Hydraulic fracturing method

ABSTRACT

Improvements in hydraulic fracturing of underground porous formations penetrated by a well bore are accomplished by the use of fracturing fluids comprising a gelled solution of a cellulose ether, e.g., carboxymethyl cellulose.

[iiT E133: 7

United States Patent 91 Clempltt HYDRAULIC FRACTURING METHOD 75 RichardL. Clampitt, Bartlesville,

Okla.

Inventor:

[73] Assignee: Phillips letrolenirKCdmpany, Bar

tlesville, Okla.

Feb. 9,1972

[21] Appl. No.: 224,921

[52] US. Cl. ..166/283, 166/307, 166/308, 252/855 R [51] Int. Cl ..E2lb43/26, E21b 43/27 [58] FleldofSear-eh ..166/283, 282,281, 166/280, 294,271, 270, 308, 307; 252/855 References Cited UNITED STATES PATENTS3/1962 Jones ..166/283 [451 Apr. 17, 1973 Primary Examiner-Stephen J.Novosad Attorney-Quigg & Oberlin ABSTRACT Improvements in hydraulicfracturing of underground porous formations penetrated by a well boreare accomplished by the use of fracturing fluids comprising a gelledsolution of a cellulose ether, e.g., carboxymethyl cellulose.

25 Claim, No Drawings HYDRAULIC FRACTURING METHOD This invention relatesto hydraulic fracturing.

Hydraulic fracturing of subterranean formations penetrated by a borehole has been widely employed for increasing the production ofhydrocarbon fluids, e.g., crude oil, natural gas, from said formations.Hydraulic fracturing comprises the injection of afracturing fluid down awell penetrating a formation, and into said fonnation under sufficientpressure to overcome the pressure exerted by the overburden. Thisresults in creating a fracture in said formation which facilitates flowof hydrocarbons through the formation and into the well.

Desirable properties of a hydraulic fracturing fluid include highviscosity, low fluid loss, low friction loss during pumping into thewell, stability under the conditions of use such as in high temperaturedeep wells, and ease of removal from the fracture and well after theoperation is complete. it would be desirable to have a fracturing fluidpossessing all of these properties.

Higher viscosities for the fracturing fluid aids in producing widerfractures. This is particularly advantageous when a viscous solution isused as a pad" preceding the acid in combination fracturing-acidizingoperations. More viscous solutions also aid in carrying propping agentsinto the formation when propping agents are used. The common thickeneragents of the prior art such as the natural gums (guar gums, etc.) andstarches require excessive amounts for worthwhile viscosity increases.Furthermore, solutions of said gums and starches are not viscositystable at the higher temperatures encountered in deeper wells, e.g.,above about 200 F.

The fluid loss properties of the fracturing fluid must be low enough topermit build-up and maintenance of the pressures necessary to fracturethe formation. Otherwise, low penetration and/or ineffective fractureswill be obtained. Various fluid loss control agents have been proposedin the past for use with various fracturing fluids. However, at best,the use of such fluid loss control agents is an undesirable complicatingfactor in the preparation and use of fracturing fluids. It would bebetter to have a fracturing fluid which does not require the use ofafluid loss control agent.

Low friction loss is desirable so as to avoid excessive well headpressures in pumping the fracturing fluid through the casing and tubingand then into the formation. Otherwise, the frictional losses can becomeprohibitive.

Stability underconditions of use, e.g., retention of sufficientviscosity at temperatures in the order of 200 F. and higher for a periodof time sufficient to carry out the fracturing operation, isparticularly important when the formations penetrated by deep hightemperature wells are being fractured. Fracturing fluids prepared frommany of the prior art thickener materials have little more viscositythan the viscosity of water at temperatures of 200 F., and higher.

The ease of removal of the fracturing fluid from the formation is highlyimportant. One disadvantage of using many highly viscous solutions isthat they are difficult to remove from the pores or the fracture afterthe operation is completed. Other high viscosity solutions sometimesleave a clogging residue in the pores of the formation. This inhibitsproduction and often requires costly clean-up operations after thefracturing operation is completed. It would be desirable to have athickened solution which would break down to a lesser viscosity within ashort time after the fracturing job is complete.

The present invention provides a solution for the above-discussedproblems. The present invention provides methods of fracturing porousformations employing aqueous gels prepared by gelling solutions ofcellulose ethers as described further hereinafter. As shown by theexamples given hereinafter, said aqueous gels have all theabove-described desirable properties.

Thus, according to the invention, there is provided a method offracturing a subterranean porous formation penetrated by a wellbore,which method comprises injecting down the well and into said formation,at a pressure sufficient to fracture the formation, a fracturing fluidcomprising an aqueous gel, and wherein said gel comprises water to whichthere has been added: a water-thickening amount of a water-solublecellulose ether; a sensible amount of a water-soluble compound ofapolyvalent metal wherein the metal present is capable of being reducedto a lower polyvalent valence state and which is sufficient to gel saidwater when the valence of at least a portion of said metal is reduced tosaid lower valence state; and an amount of a watersoluble reducing agentwhich is effective to reduce at least a portion of said metal to saidlower valence state.

Thus, one embodiment of the invention comprises using said aqueous gelsas the fracturing fluid. In the practice of the invention, the aqueousgels can be injected down the well and into the porous formationemploying conventional pumping equipment and procedures. if desired, thefracturing fluids used in the practice of the invention can be injectedinto a selected portion or portions of the porous formation. Saidselected portion(s) of the formation can be isolated by employing one ormore well packers at proper locations using packers and methods known inthe art.

The amount of said fracturing fluid injected into the formation willdepend upon the type of formation being treated, the thickness of theformation, the depth or penetration of fracturing desired, etc.Generally speaking, the use of any suitable amount is within the scopeof the invention. Thus, the invention is not limited to the use of anyparticular amount of said aqueous gels as the fracturing fluid. Amountsused in using other fracturing fluids known to the art can be used.Thus, the amount of fracturing fluid can include any amount from 1 to2,000, or more, gallons per vertical foot of formation.

Another embodiment of the invention comprises a combinationfracturing-acidizing treatment. This embodiment of the invention isparticularly useful where the formation is susceptible to attack by theacid. In this combination method of the invention the aqueous gels ofcellulose ether solutions are used as fracturing pads and are injected,prior to injection of the acid, at sufficient pressure to create thefracture. The acid is subsequently injected to react with, etch, androughen the fracture faces to'provide good conductivity when theoperation is completed.

In combination fracturing-acidizing treatments it is highly desirablethat good penetration of the acid into the formation and good etching ofthe fracture faces be obtained. This is a problem under almost allcircumstances. The severity of the problem increases as the welltemperature increases because the acid reactivity with the formationincreases with temperature. This results in a reduction in the amount oflive acid penetration. Acid penetration can also be reduced by leak-offat the fracture faces. The acid will naturally react in some of thepores adjacent to the fracture. in extreme cases there may be so-calledworm holing perpendicular to the fracture face. Another cause of acidleak-off is the presence of natural fractures in the formation beingtreated.

The aqueous gels used in the practice of the invention are particularlywell suited to be used as a fracturing pad in combinationfracturing-acidizing treatments. Said gels serve several purposes. Theyreduce the apparent acid reaction rate by reducing contact rate. Saidaqueous gels prepared from cellulose ethers are superior to otheraqueous gels in wetting and adhering to oilcovered sands. Thus, when theaqueous gel pad is displaced by the acid a thin film will remainsufficiently long to retard the acid reaction rate an amount sufficientto obtain greater penetration. The acid soon destroys the film of geland performs its intended function of etching and rough ening thefracture faces, but not before its action has been retarded sufficientlyto permit a greater quantity of live acid to penetrate further into thefracture.

Another valuable purpose of the viscous aqueous gels used in thepractice of the invention is that they increase fracture width andlength. This provides a greater fracture face for the acid to work on,resulting in fractures having greater conductivity. Fracture width isdependent to a large extent upon the viscosity of the fracturing fluid.As shown in the examples given hereinafter, the aqueous gels used in thepractice of the invention have superior high temperature viscosityproperties when compared to commercially available gels. These superiorviscosity properties result in superior fractures. Still anotheradvantage of the superior viscosity properties is that said gels willcarry more and larger propping agents in those embodiments of theinvention where propping agents are employed.

Still another purpose served by said aqueous gels is that they serve tocool the well piping during injection, thereby overcoming the limitationof corrosion inhibitors used in the acid.

Propping agents which can be used in the practice of the inventioninclude any of those known in the art, e.g., sand grains, walnut shellfragments, tempered glass beads, aluminum pellets, and similarmaterials. Generally speaking, it is desirable to use propping agentshaving particle sizes in the range of8 to 40 mesh (U.S. Sieve Series).However, particle sizes outside this range can be employed. Proppingagents are generally not used in the combination fracturing-acidizingtreatments described herein. However, it is within the scope of theinvention to use propping agents in said combination treatment. Whenpropping agents are so used they should be made of materials which arenot severely attacked by the acid used.

Acids useful in the practice of the invention include any acid which iseffective in increasing the flow of hydrocarbons through the formationand into the we!!. Examples of acids which can be used include inorganicacids such as hydrochloric acid, nitric acid, and sulfuric acid; organicacids such as acetic acid, and formic acid; and combinations ofinorganic and organic acids. The concentration or strength of the acidcan vary depending upon the type of acid, the type of formation beingtreated, and the results desired in the particular treating operation.For example, when hydrochloric acid solution is being used in apredominantly limestone formation, the concentration can vary from about5 to about 38 weight percent HCl, with concentrations within the rangeof 10 to 30 weight percent usually preferred. Organic acids are usuallyused in lower concentrations, e.g., about 10 weight percent. Onepreferred mixture of inorganic acids and organic acids comprisesmixtures of hydrochloric acid and acetic acid. For example, 15 percenthydrochloric acid solution containing sufficient acetic acid to bringthe total acidity to about 20 to 22 percent, based on equivalent HCl..The acids used in the practice of the invention can contain any of theknown corrosion inhibitors, deemulsifying agents, sequestering agents,surfactants, friction reducers, etc., known in the art. The amount ofacid used in any particular instance will depend upon a number offactors including the size or amount of formation to be treated, thetype of formation being treated, the type of acid, the concentration ofthe acid, and the formation temperature. Thus, the invention is notlimited to using any particular amount of acid in the combinationfracturing-acidizing embodiment of the invention. Any suitable amountfrom about 1 to 750, or more, gallons of acid per vertical foot offormation can be used.

The fracturing operation in accordance with the invention can be carriedout in one or more stages. A stage can comprise the following steps. Ifdesired, depending upon the well conditions, the injection of theaqueous gel can be preceded by a small slug of clean-up acid to removescale, wax deposits, etc., and clean the perforations. This clean-upacid injection can be followed with a preflush of water to cool thecasing and the formation. The aqueous gel is then injected. Usually, theacid injection follows the injection of the aqueous gel. The acid slugis then followed with an overflush of water to displace the acid. Thesecond, and any succeeding stages, can comprise the same steps carriedout in the same order.

However, it is to be understood the invention is not to be limited tothe above combination of steps. Thus, in the embodiments of theinvention comprising injecting a gelled solution of a cellulose ether asthe fracturing fluid, the only essential step is the injection of theaqueous gel under sufficient pressure to create the fracture. Theinjection of the aqueous gel can be preceded by any suitable preflushinjection steps, etc., and can be followed by any suitable subsequentoverflush or other clean-up steps. Similarly, in the combinationfracturingacidizing method of the invention the only essential steps arethe injection of the aqueous gel and the subsequent injection of theacid. Generally speaking, in said combination treatment it is preferredto inject the acid immediately following the injection of the aqueousgel fracturing fluid. However, it is within the scope of the inventionto inject a slug of water or other spacer liquid between the slug ofaqueous gel and the slug of acid.

In general, any of the water-soluble cellulose ethers can be used toprepare the aqueous gels used in the practice of the invention. Saidcellulose ethers which can be used include, among others: the variouscarboxyalkyl cellulose ethers, e.g., carboxyethyl cellulose and'carboxymethyl cellulose (CMC); mixed ethers such as carboxyalkylhydroxyalkyl ethers, e.g., carboxymethyl hydroxyethyl cellulose (CMHEC);hydroxyalkyl cellulose s such as hydroxyethyl cellulose, and hydroxypropyl cellulose; alkylhydroxyalkyl celluloses such asmethylhydroxypropyl cellulose; alkyl celluloses such as methylcellulose, ethyl cellulose, and propyl cellulose; alkylcarboxyalkylcelluloses such as ethylcarboxylnethyl cellulose; alkylalkyl cellulosessuch as methylethyl cellulose; and hydroxyalkylalkyl celluloses such ashydroxypropylmethyl cellulose; and the like. Many of said celluloseethers are available commercially in various grades. Thecarboxy-substituted cellulose ethers are available as the alkali metalsalt, usually the sodium salt. However, the metal is seldom referred toand they are commonly referred to as CMC for carboxymethyl cellulose,CMHEC for carboxymethyl hydroxyethyl cellulose, etc. For example,water-soluble CMC is commercially available in various degrees ofcarboxylate substitution ranging from about 0.3 up to the maximum degreeof substitution of 3.0. In general, CMC having a degree of substitutionin the range of 0.65 to 0.95 is preferred. Frequently, CMC havingadegree of substitution in the range of 0.85 to 0.95 is a more preferredcellulose ether. CMC having a degree of substitution less than theabove-preferred ranges is usually less uniform in properties and thusless desirable for use in the practice of the invention. CMC having adegree of substitution greater than the abovep'referred rangesusually'has a lower viscosity and more is required in the practice ofthe invention. Said degree of substitution of CMC is. commonlydesignated in practice as CMC-7, CMC-9, CMC-l2, etc., where the 7, 9,and I2 refer to a degree of substitution of0.7, 0.9, and 1.2,respectively.

In the above-described mixed ethers, it is preferred that the portionthereof which contains the carboxylate groups be substantial instead ofa mere trace. For example, in CMHEC it is preferred that thecarboxymethyl degree of substitution be at least 0.4. The degree of Ihydroxyethyl substitution is less important and can vary widely, e.g.,from about 0.l or lower to about 4 or higher.

The amount of cellulose ether used in preparing the aqueous gels used inthe practice of the invention can vary widely depending upon theviscosity grade and purity of the ether, and properties desired in saidaqueous gels. In general, the amount of cellulose ether used will be awater-thickening amount. i.e., at least an amount which willsignificantly thicken the water to which it is added. For example,amounts in the order of 25 to I00 parts per million weight (0.0025 to0.0! weight percent) have been found to significantly thicken water.Water containing 25 ppm of CMC has a viscosity increase of about 2lpercent. At 50 ppm the viscosity increase is about 45 percent. At lOOppm the viscosity increase is about l95 percent. Generally speaking,amounts in the range of from 0.0025 to 20, preferably from 0.01 to 5,more preferably 0.025 to 1, weight percent, based on the weight ofwater, can be used. However, amounts outside said ranges can be used. Ingeneral, with the proper amounts of polyvalent metal and reducing agent,the amount of cellulose ether used will determine the consistency of thegel obtained. Small amounts of cellulose ether will usually produceliquid mobile gels which can be readily pumped whereas large amounts ofcellulose ether will usually produce stiff rigid gels. If desired, saidstiff gels can be thinned" by dilution to any desired concentration ofcellulose ether. This can be done by mechanical means, e.g., stirring,pumping, or by means ofa suitable turbulent inducing device to causeshearing, such as a jet nozzle. Thus, there is really no fixed upperlimit on the amount of cellulose ether which can be used. However, whena liquid mobile gel is desired, it is preferred to dilute the moreconcentrated gels before they become rigid.

Metal compounds which can be used in preparing the aqueous gels used inthe practice of the invention are water-soluble compounds of polyvalentmetals wherein the metal is present in a valence state which is capableof being reduced to a lower polyvalent valence state. Examples of suchcompounds include potassium permanganate, sodium permanganate, ammoniumchromate, ammonium dichromate, the alkali metal chromates, the alkalimetal dichromates, and chromium trioxide. Sodium dichromate andpotassium dichromate, because of low cost and ready availability, arethe presently preferred metal-containing compounds. The hexavalentchromium in said chromium compounds is reduced in aim to trivalentchromium by suitable reducing agents, as discussed hereinafter. In thepermanganate compounds the manganese is reduced from +7 valence to +4valence as in MnO,.

The amount of said metal-containing compounds used in preparing theaqueous gels used in the practice of the invention will be a sensibleamount, i.e., a small but finite amount which is more than incidentalimpurities, but which is effective or sufficient to cause subsequentgellation when the metal in the polyvalent metal compound is reduced toa lower polyvalent valence state. The lower limit of the concentrationof the starting metal-containing compound will depend upon severalfactors including the particular type of cellulose ether used, theconcentration of the cellulose ether in the water to be gelled, thewater which is used, and the type of gel product desired. For similarreasons, the upper limit on the concentration of the startingmetal-containing compound also cannot always .be precisely defined.However, it should be noted that excessive amounts of the starting metalcompound, for example +6 chromium, which can lead to excessive amountsof +3 chromium when there is sufficient reducing agent present to reducethe excess +6 chromium, can adversely affect the stability of the gelsproduced. As discussed further hereinafter in the examples, thisprovides one valuable method for controlling gel stability so as toobtain gels which will break down with time and/or temperature to permitready well clean-up subsequent to a fracturing operation. As a generalguide, the amount of the starting polyvalent metal-containing compoundused in preparing aqueous gels in accordance with the invention will bein the range of from 0.05 to 60, preferably 0.5 to 30, weight .percentof the amount of the cellulose ether used.

Stated another way, the amount of the starting polyvalentmetalrcontaining compound used will usually be an amount sufficient toprovide at least about 3 X 10*, preferably at least 3 X l gram atoms ofsaid metal capable of being reduced per gram of cel lulose ether.Preferably, the amount of said metal capable of being reduced which isused will not exceed 4 X more preferably 2 X 10', gram atoms of saidmetal per gram of cellulose ether. However, in some situations it may bedesirable to use amounts of the starting polyvalent metal-containingcompound which are outside the above ranges. Such use is within thescope of the invention. Those skilled in the art can determine theamount of starting polyvalent metal-containing compound to be used bysimple experiments carried out in the light of this disclosure. Forexample, I have discovered that when brines such as are commonlyavailable in producing oil fields, are used as the water in preparinggels for use in the practice of the invention, less of the startingpolyvalent metal-containing compound is required than when distilledwater is used. Stable gels have been prepared using brines having a widerange of dissolved solids content, e.g., from 850, 1,200, 6,000, and90,000 ppm dissolved solids. Gellation rates are frequently faster whenusing said brines. Such oil field brines commonly contain varyingamounts of sodium chloride, calcium chloride, magnesium chloride, etc.Sodium chloride is usually present in the greatest concentration. Theword water" is used generically herein and in the claims, unlessotherwise specified, to include such brines, fresh water, and otheraqueous media which can be gelled in accordance with the invention.

Suitable reducing agents which can be used in preparing the aqueous gelsused in the practice of the invention include sulfur-containingcompounds such as sodium sulfite, sodium hydrosulfite, sodiummetabisulfite, potassium sulfite, sodium bisulfite, potassiummetabisulfite, sodium sulfide, sodium thiosulfate, ferrous sulfate,thioacetamide, and others; and nonsulfur-containing compounds such ashydroquinone, ferrous chloride, p-hydrazinobenzoic acid, hydrazinephosphite, hydrazine dichloride, and others. Some of the above reducingagents act more quickly than others, for example, sodium thiosulfateusually reacts slowly in the absence of heat, e.g., requiring heating toabout l25l 30 F. The presently most preferred reducing agents are sodiumhydrosulfite, potassium hydrosulfite, sodium thiosulfate, and potassiumthiosulfate.

The amount of reducing agent to be used in preparing the aqueous gelsused in the practice of the invention will be a sensible amount, i.e., asmall but finite amount which is more than incidental impurities, butwhich is effective or sufficient to reduce at least a portion of thehigher valence metal in the starting polyvalent metal-containingcompound to a lower polyvalent valence state. Thus, the amount ofreducing agent to be used depends, to some extent at least, upon theamount of the starting polyvalent metal-containing compound which isused. In many instances, it will be preferred to use an excess ofreducing agent to compensate for dissolved oxygen in the water, exposureto air during preparation of the gels, and possible contact with otheroxidizing substances such as might be encountered in field operations.As a general guide, the amount of reducing agent used will generally bewithin the range of from 0.1 to at least 150, preferably at least about200, weight percent of the stoichiometric amount required to reduce themetal in the starting polyvalent to said lower polyvalent valence state,e. g., +6 Cr to +3 Cr. However, in some instances, it may be desirableto use amounts of reducing agent outside said ranges. The use of suchamounts is within the scope of the invention. Those skilled in the artcan determine the amount of reducing agent to be used by simpleexperiments carried out in the light of this disclosure.

Various methods can be used for preparing the aqueous gels used in thepractice of the invention. Either the metal-containing compound or thereducing agent can be first added to a solution of the cellulose etherin water or other aqueous medium, or said metal-contain ing compound andsaid reducing agent can be added simultaneously to the solution oraqueous medium con taining the cellulose ether. Generally speaking,where convenient, the preferred method is to first disperse thecellulose ether in the water or other aqueous medium. Themetal-containing compound is then added to the solution or aqueousmedium containing the cellulose ether and the reducing agent, withstirring. The reducing agent is then added to the dispersion ofcellulose ether, with stirring. Gellation starts as soon as reduction ofsome of the higher valence metal in the starting polyvalentmetal-containing compound to a lower valence state occurs. Thenewly-formed lower valence metal ions, for example +3 chromium obtainedfrom +6 chromium, effect rapid crosslinking of the cellulose ethers andgellation of the solution or aqueous medium containing same.

It is also within the scope of the invention to prepare a dry mixture ofthe cellulose ether, the metal-containing compound and the reducingagent, in proper proportions, and then add this dry mixture to theproper amount of water.

An advantage of the invention is that ordinary ambient temperatures andother conditions can be used in practically all instances in preparingthe aqueous gels or aqueous mediums containing same. However, in someinstances, a small amount of heat may be desirable to aid in theformation of the gel, e.g., heating to a temperature of about l25l 30 F.

Aqueous gels can be prepared having a wide range of viscosities orfirmness ranging from low viscosity or highly mobile gels having arelatively low viscosity up to firm or rigid gels which are nonmobile.The choice of gel viscosity or concentration will depend upon the use tobe made of the gel. The actual viscosity and/or gel strength of the gelwill depend upon the type and concentration of the cellulose ether, thetype and amount of starting polyvalent metal compound used, and the typeand amount of reducing agent used.

One procedure which can be used to prepare said gels is to prepare arelatively concentrated or high viscosity gel and dilute same to aviscosity or concentration suited for the actual use of the gel. In manyinstances, this procedure results in a more stable gel. This should betaken into consideration since in the practice of the present inventionhighly stable gels are not, generally speaking, desirable. Gels havinggood initial stability sufficient to permit pumping and placement in theformation to fracture same, but which will break down with time and/ortemperature to permit easy well clean-up are most useful in the practiceof the present invention. Generally speaking, it is preferred that saidgels have a stability, e.g., viscous life, within the range of about 15minutes to about 12 hours.

When employing said dilution technique a starting solution of celluloseether containing, for example, 1,000 to 10,000 ppm (0.1 to 1 wt.percent), or more, of cellulose ether can be used. This solution is thengelled by the addition of suitable amounts of polyvalent metal compoundand reducing agent. After gellation has proceeded to the desired'extent,the resulting gel can be diluted to the concentration or viscosity-mostsuited for its intended use. The more concentrated cellulose ethersolutions usually have a faster rate of gellation. Thus, in mostinstances, it will be preferred to carry out the dilution soon after thecomponents of the gel have been added to the water or other aqueousmedium, e.g., within about 5 to 30 minutes. Preferably, theconcentration of the cellulose ether in the concentrated gel" will be atleast twice that in the final gel. Dilution of the gel retards the rateof gellation. Thus, this dilution technique can be employed to controlthe gellation rate, if desired. One advantage of said dilution techniqueis that it is usually more convenient to weigh out and handle the largerquantities of reagents.

Generally speaking, the pH of the final solution of the gelling reagentsis preferably less than 7, more preferably in the order of 6. Ingeneral, pH is not controlling, but higher pH values retard gellationrate. In general, the pH of the gelling solution will depend upon thereducing agent used. If desired, the pH can be adjusted by the additionof a suitable acid, depending upon the reducing agent used.

Herein and in the claims, unless otherwise specified, the aqueous gelsused in the practice of the invention are defined for convenience, andnot by way of limitation, in terms of the amount of cellulose ethercontained therein, irrespective of whether or not all the celluloseether is crosslinked. For example, a 1 weight percent or 10,000 ppm gelis a gel which was prepared from a starting cellulose ether solutionwhich contained 1 weight percent or 10,000 ppm by weight of celluloseether. The same system is employed for the gels prepared by theabove-described dilution technique.

The following examples will serve to further illustrate the invention.

EXAMPLE 1 It is the purpose of this example to demonstrate that gels ofCMC solutions exhibit high viscosities at relatively high temperatures.A solution was prepared by mixing 2.4 grams of CMC9 in 500 cc ofBartlesville, Oklahoma, tap water to form a 4800 ppm (by weight)solution. To said solution there was added 7.5 cc of a 10 percent weightsolution of sodium dichromate ICr2O1'2H2O), followed by mild stirring.(The solutions were at room temperature 75 F.). Following the additionof the sodium dichromate, the solution was placed in a water bath andthe temperature thereof increased to 130 F. over a period of 5 to 10minutes. When the temperature reached 130' F., 23.5 cc of a percent byweight solution of sodium thiosulfate (Na.S O5H Q), a reducing agent,was added to initiate gellation, followed by hand stirring for 60seconds. A portion was then poured into a stainless steel cup of a highpressure, high temperature Model 50 Fann Viscometer so the rate ofgellation with temperature and time could be monitored. Theconcentration of the final ingredients, considering dilution due to theaddition of the final two chemicals, follows:

Concentration, Component ppm by weight CMC-9 4520 Sodium Dichromate(Na,Cr,O -2H,O) 1410 Sodium Thiosulfate (Na,S,O,-5 H,O) 8850 The rate ofgellation, as exhibited by increased viscosity, was measured as thefinal mixture was heated by the oil bath surrounding the holding cup ona standard Model 50 Fann Viscometer. The data in Table i show theviscosity increased from 18.5 centipoises at a shear rate of 500 sec"after the addition of the last component to a peak viscosity ofcentipoiaes at a temperature of 237 F. and then slowly decreased as thetemperature was raised to 303 F. The heat-up rate was an approximatesimulation of heat-up rates which occurred in a hydraulically-inducedfracture in fracture-acidizing treatments of deep Ellenburger gas wellsin the Gomez field in Pecos County, Texas. The example clearlydemonstrates a technique for preparing viscous gels useful as fracturingfluids.

TABLE I Viscosity of Gelled CMC Solution Temperature of Time FollowingGelled CMC Viscosity of Gelled Addition of Last Solution in CMCSolution,

Centipoise Gelling Component, Fann Viscometer, at Shear at Shear MinutesF. Rate of Rate of 170 sec 500 lee" 8 Not Measured 18.5 10.5 137 25 1333 15 44 16.5 50 18 60 i 20 74 22 87 24 102 26 203 108 28 21 B 112 29225 198 N.M. 31 237 Not Measured 1'15 32 243 l 10 32.5 250 I80 N.M. 34255 Not Measured 100 35.5 265 90 37 275 120 77 39 283 Not Measured 60 41293 40 42 300 35 43 303 39 N.M.

Measured by a high temperature, high pressure, Fann Model 50 RotationalViscometer at a pressure of 500 psi.

EXAMPLE II It is the purpose of this example to demonstrate that gels ofCMC solutions will form while the mixture is being pumped or circulatedin turbulent flow and to present friction loss characteristics useful incalculating pressure drops which will be obtained in tubular goodsduring fracture treatments where said gels are used.

A quantity of CMC-9 was added through an eductor to Duncan, Oklahoma,tap water in a 250 gallon Halliburton ribbon blender. The final CMCconcentration was 40 pounds per thousand gallons or approximately 4800parts per million based upon the weight of the water. The solution wasmaintained at 130 F. constant temperature throughout this preparation.When the CMC solution reached full hydration as indicated by a 300 rpmreading on a Model 35 Fann V-G meter (viscometer), sodium dichromate(Na- Cr O '2H O) was added to the CMC solution in an amount sufficientfor a concentration of 12 pounds per thousand gallons of CMC solutionwhich is approximately equivalent to 1440 parts per million. The sodiumdichromate was readily soluble and went into solution in less thanminutes in the ribbon blender tank. This solution was used as a basesolution in the tests described below.

a. Friction loss tests were run on a portion of said base solution whilecirculating same in turbulent flow, under controlled conditions oftemperature and velocity, through a nominal one-inch diameter pipe loophaving a test section 40 feet in length. Pressure drop readings weretaken during said calculation. The data obtained were employed tocalculate the friction loss which could be expected in pumping said basesolution into a well under the following conditions: well depth 22,500feet; casing 17,000 feet of 5%; inch, 23 lb./ft. steel casing and 5,500feet of 5 inch, 23.2 lb./ft. steel casing; surface injection temperatureof 130 F.; a formation temperature of 350 F. at 22,500 feet; and pumpingrates of 25 and 30 barrels per minute. These calculated data are setforth in Table ll below.

b. Friction loss tests, and rate of gellation measurements, were run onanother portion of said base solution in which gellation was effectedwhile the base solution was being circulated as described in paragraph(a) above. Gellation was effected by adding to the circulating solutionsufficient sodium thiosulfate, Na,S,O -5l-l,O, (in solution) to provide1440 ppm by weight in the circulating solution. Pressure drop readingswere taken during said circulation. The data showed that gellation wasretarded for about 370 seconds before the gelled solution showed anygelled characteristics which could increase friction losses over anungelled system. It was concluded this system could properly be called aretarded gelling system since no significant amount of gellationoccurred for about 6 minutes. Friction loss for the well conditionslisted in paragraph (a) above was calculated from the data obtained.These calculated data are set forth in Table ll below.'

c. Friction loss tests, and rate of gellation measurements, were run onanother portion of said base solution, as described in paragraph (b)"above, except that gellation was effected by adding to the circulatingsolution sufficient sodium hydrosulfite, Na,S,O (in solution) to provideI440 ppm by weight in the circulating solution. Pressure drop readingswere taken during said circulation. The data showed that gellationbegain within 60 seconds following addition of the sodium hydrosulfite.This system would not be considered a retarded system at the temperaturetested (approx. 130 F.). Friction loss for the above-described wellconditions were calculated from the data obtained. See Table 1] below.

d. Friction loss tests were also run on Duncan, Oklahoma, tap waterwhile being circulated as described in paragraph (a) above. Frictionloss for the well conditions listed in paragraph (a) above wascalculated from the data obtained. These calculated data are set forthin Table [I below.

TABLE 11 Comparison of Calculated Friction Loss Values Injection Rate,Bbls./minute Total Pressure Drop, psi Fresh Water (d) The above testsand data show that solutions of CMC are friction-reducing agents ascompared to plain water. Gelled solutions of CMC gelled during pumpingare also friction-reducing agents as compared to water because they canbe pumped at high rates down casing in a well with less horsepower thanis required for pumping water. Gelling of the CMC solution duringpumping is presently preferred because it affords another measure ofcontrol over gellation rate. The abovedescribed retarded system usingsodium thiosulfate as the reducing agent affords further control overgellation rate.

Calculated values for a comparable solution of CMC fully gelled" priorto pumping are 3050 and 3677 psi total pressure drop at pumping rates of25 and 30 barrels per minute, respectively, in the above-described22,500 feet string of pipe. This shows the advantage of gelling duringpumping.

The above example also illustrates one presently preferred method ofcarrying out a fracturing operation. Said method comprises preparing abase fracturing fluid comprising a solution of a cellulose ether, ad-

added thereto. It is also within the scope of the invention toincorporate all the components of the aqueous gel into a stream of waterwhile it is being pumped, e.g., into a well. For example, CMC could beadded first to the flowing stream of water and the other componentsadded subsequently in any suitable order. Turbulent flow conditions inthe pipe will provide proper mixing.

EXAMPLE III This example is an illustration of the breakdown of gelledsolutions of CMC upon exposure to elevated temperatures. Gelledsolutions of sodium carboxymethyl cellulose (CMC-7 and CMC-9) wereprepared to contain about 5000 ppm by weight of CMC, about 1500 ppm ofNa',s,o,, and about 1500 ppm of Na,Cr,O 21-1 0. The gelled solutionswere prepared by first dispersing the CMC in water and then addingthereto solutions of the Na,S,O and Na Cr O with stirring. Samples ofsaid gelled solutions were then subjected to elevated temperatures inthe order of 300, and 350 F., for varying periods of time, underpressures in the order of 15,000 to 20,000 psi in an Amoco cementconsistometer. Viscosity determinations were then run in a Model 35 FannVG meter after the samples had cooled. From these viscositydeterminations it was concluded that the gelled solutions wereself-breaking at elevated temperatures, and that the breaking timedecreases with increases in temperature. For example, said viscositydeterminations indicated that the break time of the gelled solutions ofCMC studied in this example was in the order of 8 to 12 hours at 300F.,and 2 to 3 hours at 350 F. These data indicate that gelled solutions ofCMC can be used as fracturing fluids in high temperature formations, andthat said gels will break down after a relatively short time to permiteasy well clean-up.

EXAMPLE lV There are several methods by which gelled solutions ofCMC canbe caused to break down with time so their final viscosity approximatesthat for ungelled CMC solution, or water. One method is to use excessiveamounts of the gelling agents which will produce adequate viscosity andgel strengths required during the fracture treatment, but which willsubsequently cause breakdown to a thin solution, allowing rapid wellclean-up following a fracture treatment. The data in Table IV below setforth results of three gelled solutions of CMC-9 prepared in essentiallythe same manner, but gelled with differing amounts of gelling agents.Solution No. 3 gelled the fastest and also broke down more quicklybecause of an excessive amount of gelling agents. Solution No. l, gelledwith about onesixth the amount of gelling agents used with Solution No.3, required about 18 minutes to reach maximum viscosity before it beganto break down. Significantly less gelling agents are required to gelsolutions of CMC made with hard brines than are required to gelsolutions made with fresh water. As the salinity of the water increases,the rate of gellation of CMC solutions increases. The life-span" ofgelled solutions of CMC can be controlled by tailoring the amounts ofgelling agents in accordance with the water and temperature conditionsthat will be encountered.

TABLE IV Efl'ect of Concentration of Gelling Agents Upon Viscosity ofGelled CMC Solutions Time after addition of last gelling ApparentViscosity, centipoise at 170 sec" agent, minutes Solution SolutionSolution No. I No. 2 No. 3

0.25 N.M."' N.M. 81.0

0.50 N.M. N.M. N.M.

1.0 N.M. N.M. 46.5

8.0 81.0 N.M. N.M.

13.0 96.0 N.M. N.M.

18.0 120.0 N.M. N.M.

32.0 N.M. 21.0 N.M. 52.0 39.0 N.M. N.M. 79.0 24.0 15.0 N.M. 218 18.0 N.N.M.

(l) N.M. not measured The gelled solutions used to obtain the data inthe above Table IV were prepared as follows. To one liter of actualproduced brine from the North Burbank Unit (Burbank, Oklahoma totaldissolved solids approximates 90,000 ppm) there was added 5 grams ofCMC-9 to form a 5000 ppm solution. The solution was stirred 5 minutes ona malt mixer and allowed to stand for 30 minutes to reach maximumviscosity of 21.6 centipoises at a shear rate of 170 see, as measuredwith a Model 35 Fann VG meter at a temperature of F. The one-litersolution was then split into three equal portions, designated Solutions1, 2, and 3, and placed in cups on Fann viscometers. The solutions werestirred continuously at rpm which is equivalent to a shear rate of 170sec. Each of the solutions was gelled by first adding sodium dichromateand then adding sodium hydrosulfite so that the final solutions had thefollowing compositions:

Solutions Ingredients 1 2 3 CMC, ppm 5000 5000 5000 Na,Cr,O -2H,O, ppm500 1000 3000 Na,S,O ppm 500 1000 3000 EXAMPLE V Different, andexcessive, amounts of gelling agents were used to prepare gelledsolutions of CMC-9 using a relatively fresh water (total dissolvedsolids of 1100 parts per million). A base CMC solution was prepared bymixing 3.57 grams of CMC in one liter of said water at room temperatureon a multimixer. Concentration of the CMC approximated 3570 parts permillion. The resulting solution was divided into three equal portions byvolume. Two portions were gelled using two dif ferent reducing agents,and sodium dichromate, and the other portion was used for base viscositycomparisons. Table V below shows that high viscosities were developed inless than 5 hours for the'two gelled solutions, and the gels eventuallybroke down to lower level viscosities within 7 days. i

TABLE V Effect of Reducing Agents and Metal Salts on Gels of CMCSolutions Apparent viscosities After Aging at Room Temperature (3570 ppmCMC) (715 ppm of C,H,O,) (l ppm of Na,Cr,O-,-2H,O)

The data in the above Examples l-V show that the rate of gellation ofsolutions of CMC, and/or the lifespan of the resulting gel, can betailored in accordance with conditions encountered in the field. Thiscan be done be taking into consideration the temperature of theformation where the gel is to be used, the amount of gelling agents usedin preparing the gels, and the water used in preparing the gels. Thus, agel can be tailored to have a life-span of 12, 8, 4, or even 2 hours, orless, so that the gel will break down to a viscosity approaching that ofwater within the time selected. This will permit ready well clean-upafter the fracturing treatment and permit ready removal of the gelresidue, such as by producing of formation fluids. Gels can be preparedwhich will break back to viscosities of less than about centipoises, oreven to the viscosity of water, by proper consideration of theabove-mentioned factors.

EXAMPLE VI Four fracturing fluids were tested for high temperatureviscosity properties. These fluids were: 1) a gelled solution of CMC-9,concentration 4800 ppm by weight; (2) a gelled solution of CMC-9,concentration 3600 ppm by weight; (3) a gelled solution of guar gum(commercially available), concentration 9600 ppm by weight; and (4) anungelled solution of a commercially available modified guar gum.

Fluid (1) was prepared by dry blending predetermined portions of CMC-9and Na Cr O '2H O. The resulting blend was then added to 5 liters ofDuncan, Oklahoma, tap water, with stirring on a Waring blender at lowspeed for 20 minutes. The resulting solution contained 4800 ppm byweight of CMC and 1440 ppm by weight of Na Cr O '2H O. The solution wasthen transferred to a one-quarter inch diameter pipe loop friction losstesting apparatus. This apparatus comprised a pipe loop provided withheating means and circulating means for measuring friction losses duringsimulated pumping operations. The solution was heated from roomtemperature to 130 F. while being circulated. An amount of Na S Osufficient to provide 1440 ppm by weight (based on the above S-litersolution of CMC Na,Cr,O 2l-l,O) was dissolved in 150 ml of water andadded to the circulating solution. Gellation was initiated in less thanone minute. Circulation was continued for about minutes and thenterminated. The gelled solution was permitted to set overnight and coolto room temperature. The following day the viscosity of the gel wasdetermined over the range of 70 to 300 F. employing a recording hightemperature, high pressure Model 50 Fann viscometer at a pressure of 550psi. During these viscosity measurements the temperature was increasedfrom 70 F. to 300 F. in 20 minutes.

Fluid (2) was prepared and tested in essentially the same manner asdescribed above for fluid (1), except that fluid (2) contained 3600 ppmby weight of CMC- 9, 1080 ppm by weight of Na Cr O 2H,O, and 1080 ppm byweight of Na S O During the viscosity measurements the temperature wasincreased from 70 F. to 290 F. in 17 minutes.

Fluid (3), the commercially available gelled solution of guar gum (exactcomposition not known), was prepared in a concentration of 9600 ppm byweight in Duncan, Oklahoma, tap water using the standard gelling agent(also unknown). The mixture was stirred one minute on a malt mixer. Thesolution gelled immediately at room temperature upon addition of thegelling agent. This gel is available commercially as a fracturing fluid.Viscosity determinations were made on the gel in the same manner asdescribed above. During the viscosity measurements the temperature wasincreased from F. to 300 F. in 16 minutes.

Fluid (4) was prepared by mixing a portion of the modified guar gum in asufficient portion of Duncan, Oklahoma, tap water to give a guar gumconcentration of 3600 ppm by weight. The mixture was stirred one minuteon a malt mixer. This fluid is available commercially as a fracturingfluid. Viscosity determinations were made on this fluid in the samemanner as described above. During the viscosity measurements thetemperature was increased from F. to 210 F. in 17.5 minutes.

For comparison purposes a plot of Viscosity-Centipoise at 5l l secondsshear rate vs. Temperature F. was made for each of said four fluids. Asmooth curve was obtained for each fluid. Table VI below sets forth adirect comparison of the viscosities of said four fluids at temperatureintervals of 25 F.

TABLE VI Temp.Apparcnt Viscosity cp. at 511 sec. shear rate F. Fluid l)Fluid (2) Fluid (3) Fluid (4) 75 200 125 70 l7 l82 100 43 i3 187 98 38I0 200 102 45 7 I75 202 l00 Y 46 4 225 76 i 22 250 164 62 20 275 I47 5016 300 I30 38 13 The above data clearly show the superior viscosityproperties of Fluids (1) and (2) at high temperatures.

EXAMPLE VII High temperature fluid loss values were obtained on a gelledsolution of CMC-9 containing 5000 ppm by weight of CMC, and gelled byadding thereto 1500 ppm by weight of Na Cr,O -2l-l O and 1500 ppm byweight of Na S O These fluid loss values were obtained employing aBaroid No. 387 filter press, using three Baroid 988 filter papers, andthe procedure described in AP! RP 13B, Third Edition, February 1971. Thespurt loss values were obtained by plotting fluid loss versus the squareroot of time. The spurt loss is the value of the zero time intercept ofthis plot. The m values are the slope of the plotted line. The resultsof the tests are set forth in Table Vll below.

TABLE Vll 200 F. 250' F. 300 F. I000 psi 1000 psi 1000 psi Fluid loss,ml./25 min. 28.3 38.4 46.6 Spur! Loss, ml. 0 O 1.4 m( Fluid loss Spur!Loss)/5 5.7 7,7 9.0

The above data show that gelled solutions of CMC have good fluid losscharacteristics at high temperatures.

in the practice of the invention, the fracturing fluids can be injectedinto the formation at any suitable pressures sufficient to overcome theweight of the overburden. As will be understood by those skilled in theart,

this will vary from region to region. However, generally speaking, saidpressures will be in the range of from 0.5 to 1.5 psi per foot of welldepth.

Herein and in the claims, unless otherwise specified, the term solution"is employed generically and includes colloidal solutions as well as truesolutions.

While certain embodiments of the invention have been described forillustrative purposes, the invention is not limited thereto. Variousother modifications or embodiments of the invention will be apparent tothose skilled in the art in view of this disclosure. Such modificationsor embodiments are within the spirit and scope of the disclosure.

I claim:

l. A method of fracturing a subterranean porous formation penetrated bya wellbore, which method comprises injecting down the well and into saidformation, at a pressure sufficient to fracture the formation, afracturing fluid comprising an aqueous gel, and wherein said gelcomprises water to which there has been added:

a water-thickening amount of a water-soluble cellulose ether;

a sensible amount of a water-soluble compound of a polyvalent metalwherein the metal present is capable of being reduced to a-lowerpolyvalent valence state and which is sufficient to gel said water whenthe valence of at least a portion of said metal is reduced to said lowervalence state; and

an amount of a water-soluble reducing agent which is effective to reduceat least a portion of. said metal to said lower valence state.

2. A method according to claim 1 wherein said aqueous gel compriseswater to which there has been added: from 0.0025 to 20 weight percent ofsaid cellulose ether, based upon the'weight of said water;

from 0.05 to 60 weight percent of said polyvalent metal compound basedupon the weight of said cellulose ether; and

from 0.1 to at least about 200 percent of the stoichiometric amount ofsaid reducing agent required to reduce said polyvalent metal to saidlower polyvalent valence state.

3. A method according to claim 2 wherein said cellulose ether is acarboxymethyl cellulose ether.

4. A method according to claim 3 wherein said compound of a polyvalentmetal is a compound of chromium wherein the valence of the chromium is+6 and the valence of at least a portion of said chromium is reduced to+3.

5. A method according to claim 4 wherein said chromium compound isselected from the group con sisting of ammonium chromate, ammoniumdichromate, the alkali metal chromates and dichromates, chromiumtrioxide, and mixtures thereof.

6. A method according to claim 5 wherein said reducing agent is selectedfrom the group consisting of hydroquinone, sodiurn sulfide, sodiumhydrosulfite, sodium metabisulfite, potassium sulfite, sodium bisulflte,potassium metabisulfite, sodium sulfite, sodium thiosulfate, ferroussulfate, ferrous chloride, phydrazinobenzoic acid, hydrazine phosphite,hydrazine dihydrochloride, and mixtures thereof.

' 7. A method according to claim 2 wherein:

said cellulose ether is sodium carboxymethyl cellulose;

said polyvalent metal compound is sodium dichromate; and

said reducing agent is sodium hydrosulfite.

8. A method according to claim 2 wherein:

said cellulose ether is sodium carboxymethyl cellulose;

said polyvalent metal compound is sodium dichromate; and

said reducing agent is sodium thiosulfate.

9. A method according to claim 1 wherein:

said formation is susceptible to attack by an acid;

a slug of said fracturing fluid comprising said gel is injected intosaid formation; and

a slug of an acid is injected into said formation subsequent to theinjection of said fracturing fluid.

10. A method according to claim 9 wherein a slug of a spacer fluid isinjected into said formation after the injection of said fracturingfluid and prior to injecting said acid.

11. A method according to claim 9 wherein said acid is selected from thegroup consisting of hydrochloric acid, formic acid, acetic acid, andmixtures thereof.

12. A method according to claim 11 wherein said aqueous gel compriseswater to which there has been added:

from 0.025 to 1 weight percent of said cellulose ether, based upon theweight of said water;

from 0.5 to 40 weight percent of said polyvalent metal compound, basedupon the weight of said cellulose ether; and

from 0.5 to at least about percent of the stoichiometric amount of saidreducing agent required to reduce said polyvalent metal to said lowerpolyvalent valence state.

13. A method according to claim 12 wherein:

said celluloseether is an alkali metal carboxymethyl cellulose; and

said polyvalent metal compound is an alkali metal dichromate.

14. A method according to claim 13 wherein:

said cellulose ether is sodium carboxymethyl cellulose;

' said polyvalent metal compound is sodium dichromate; said reducingagent is sodium hydrosulfite or sodium thiosulfate; and

said acid comprises a solution of hydrochloric acid.

15. A method according to claim 1 wherein the temperature of saidformation is greater than about 200 F. and the life-span of said gel issuch that it breaks down to a viscosity approaching that of water inlessthan about l2 hours.

16. A method according to claim 1 wherein an excessive amount of saidgelling agents is used, relative to the amount of said cellulose ether,so that the life-span of said gel is such that it breaks down to aviscosity approaching that of water in less than about 12 hours.

17. A method of fracturing a subterranean porous formation penetrated bya wellbore, which method comprises, in combination, the steps of:

A. forming a base fracturing fluid by adding to a given quantity ofwater from 0.025 to 1 weight percent of a water-soluble cellulose ether,based on the weight of said water;

B. adding to said base fracturing fluid one of (a) from 0.5 to 40 weightpercent of a water-soluble compound of chromium wherein the valence ofthe chromium is +6 and which is sufficient to gel said water when thevalence of at least a portion of said chromium is reduced from +6 to +3,or (b) from 0.5 to at least about 150 percent of the stoichiometricamount of a water-soluble reducing agent which is effective to reducethe valence of said chromium from +6 to +3;

C. pumping a slug of said base fracturing fluid of step (B) down saidwell and into said formation under a pressure sufficient to create afracture in said formation; and

D. during said pumping adding to said base fractursaid cellulose etheris a carboxymethyl cellulose ether; said chromium compound is selectedfrom the group consisting of ammonium chromate, ammonium.

dichromate, the alkali metal chromates and dichromates, chromiumtrioxide, and mixtures thereof; and

said reducing agent is selected from the group consisting ofhydroquinone, sodium sulfide, sodium hydrosulfite, sodium metabisulfite,potassium sulfite, sodium bisulfite, potassium metabisulfite, sodiumsulfite, sodium thiosulfate, ferrous sulfate, ferrous chloride,p-hydrazinobenzoic acid, hydrazine phosphite, hydrazine dihydrochloride,and mixtures thereof.

19. A method according to claim 18 wherein:

said cellulose ether is an alkali metal carboxymethyl cellulose; and

said chromium compound is an alkali metal dichromate.

20. A method according to claim 19 wherein:

said cellulose ether is sodium carboxymethyl cellulose;

said chromium compound is sodium dichromate; and

said reducing agent is sodium thiosulfate or sodium hydrosulfite.

21. A method according to claim 17 wherein said formation is susceptibleto attack by an acid, and said method comprises in further combinationthe step of:

E. injecting a slug of an acid into said formation subsequent to theinjection of said fracturing fluid as in said steps (C) and (D).

22. A method according to claim 21 wherein a slug of a spacer fluid isinjected into said formation after said steps (C) and (D) and prior tosaid step (E).

23. A method according to claim 21 wherein:

said cellulose ether is a carboxymethyl cellulose ether;

said chromium compound is selected from the group consisting of ammoniumchromate, ammonium dichromate, the alkali metal chromates anddichromates, chromium trioxide, and mixtures thereof;

said reducing agent is selected from the group con- 'sisting ofhydroquinone, sodium sulfide, sodium hydrosulfite, sodium metabisulfite,potassium sulfite, sodium bisulfite, potassium metabisulfite, sodiumsulfite, sodium thiosulfate, ferrous sulfate,

ferrous chloride, g-lg drazinobenzoic acid, hydrazine phosphite, y razmedlhydrochloride,

2. A method according to claim 1 wherein said aqueous gel compriseswater to which there has been added: from 0.0025 to 20 weight percent ofsaid cellulose ether, based upon the weight of said water; from 0.05 to60 weight percent of said polyvalent metal compound based upon theweight of said cellulose ether; and from 0.1 to at least about 200percent of the stoichiometric amount of said reducing agent required toreduce said polyvalent metal to said lower polyvalent valence state. 3.A methOd according to claim 2 wherein said cellulose ether is acarboxymethyl cellulose ether.
 4. A method according to claim 3 whereinsaid compound of a polyvalent metal is a compound of chromium whereinthe valence of the chromium is +6 and the valence of at least a portionof said chromium is reduced to +3.
 5. A method according to claim 4wherein said chromium compound is selected from the group consisting ofammonium chromate, ammonium dichromate, the alkali metal chromates anddichromates, chromium trioxide, and mixtures thereof.
 6. A methodaccording to claim 5 wherein said reducing agent is selected from thegroup consisting of hydroquinone, sodium sulfide, sodium hydrosulfite,sodium metabisulfite, potassium sulfite, sodium bisulfite, potassiummetabisulfite, sodium sulfite, sodium thiosulfate, ferrous sulfate,ferrous chloride, p-hydrazinobenzoic acid, hydrazine phosphite,hydrazine dihydrochloride, and mixtures thereof.
 7. A method accordingto claim 2 wherein: said cellulose ether is sodium carboxymethylcellulose; said polyvalent metal compound is sodium dichromate; and saidreducing agent is sodium hydrosulfite.
 8. A method according to claim 2wherein: said cellulose ether is sodium carboxymethyl cellulose; saidpolyvalent metal compound is sodium dichromate; and said reducing agentis sodium thiosulfate.
 9. A method according to claim 1 wherein: saidformation is susceptible to attack by an acid; a slug of said fracturingfluid comprising said gel is injected into said formation; and a slug ofan acid is injected into said formation subsequent to the injection ofsaid fracturing fluid.
 10. A method according to claim 9 wherein a slugof a spacer fluid is injected into said formation after the injection ofsaid fracturing fluid and prior to injecting said acid.
 11. A methodaccording to claim 9 wherein said acid is selected from the groupconsisting of hydrochloric acid, formic acid, acetic acid, and mixturesthereof.
 12. A method according to claim 11 wherein said aqueous gelcomprises water to which there has been added: from 0.025 to 1 weightpercent of said cellulose ether, based upon the weight of said water;from 0.5 to 40 weight percent of said polyvalent metal compound, basedupon the weight of said cellulose ether; and from 0.5 to at least about150 percent of the stoichiometric amount of said reducing agent requiredto reduce said polyvalent metal to said lower polyvalent valence state.13. A method according to claim 12 wherein: said cellulose ether is analkali metal carboxymethyl cellulose; and said polyvalent metal compoundis an alkali metal dichromate.
 14. A method according to claim 13wherein: said cellulose ether is sodium carboxymethyl cellulose; saidpolyvalent metal compound is sodium dichromate; said reducing agent issodium hydrosulfite or sodium thiosulfate; and said acid comprises asolution of hydrochloric acid.
 15. A method according to claim 1 whereinthe temperature of said formation is greater than about 200* F. and thelife-span of said gel is such that it breaks down to a viscosityapproaching that of water in less than about 12 hours.
 16. A methodaccording to claim 1 wherein an excessive amount of said gelling agentsis used, relative to the amount of said cellulose ether, so that thelife-span of said gel is such that it breaks down to a viscosityapproaching that of water in less than about 12 hours.
 17. A method offracturing a subterranean porous formation penetrated by a wellbore,which method comprises, in combination, the steps of: A. forming a basefracturing fluid by adding to a given quantity of water from 0.025 to 1weight percent of a water-soluble cellulose ether, based on the weightof said water; B. adding to said base fracturing fluid one of (a) from0.5 to 40 weight percenT of a water-soluble compound of chromium whereinthe valence of the chromium is +6 and which is sufficient to gel saidwater when the valence of at least a portion of said chromium is reducedfrom +6 to +3, or (b) from 0.5 to at least about 150 percent of thestoichiometric amount of a water-soluble reducing agent which iseffective to reduce the valence of said chromium from +6 to +3; C.pumping a slug of said base fracturing fluid of step (B) down said welland into said formation under a pressure sufficient to create a fracturein said formation; and D. during said pumping adding to said basefracturing fluid the other of said reagents (a) and (b) which was notadded thereto in said step (B).
 18. A method according to claim 17wherein: said cellulose ether is a carboxymethyl cellulose ether; saidchromium compound is selected from the group consisting of ammoniumchromate, ammonium dichromate, the alkali metal chromates anddichromates, chromium trioxide, and mixtures thereof; and said reducingagent is selected from the group consisting of hydroquinone, sodiumsulfide, sodium hydrosulfite, sodium metabisulfite, potassium sulfite,sodium bisulfite, potassium metabisulfite, sodium sulfite, sodiumthiosulfate, ferrous sulfate, ferrous chloride, p-hydrazinobenzoic acid,hydrazine phosphite, hydrazine dihydrochloride, and mixtures thereof.19. A method according to claim 18 wherein: said cellulose ether is analkali metal carboxymethyl cellulose; and said chromium compound is analkali metal dichromate.
 20. A method according to claim 19 wherein:said cellulose ether is sodium carboxymethyl cellulose; said chromiumcompound is sodium dichromate; and said reducing agent is sodiumthiosulfate or sodium hydrosulfite.
 21. A method according to claim 17wherein said formation is susceptible to attack by an acid, and saidmethod comprises in further combination the step of: E. injecting a slugof an acid into said formation subsequent to the injection of saidfracturing fluid as in said steps (C) and (D).
 22. A method according toclaim 21 wherein a slug of a spacer fluid is injected into saidformation after said steps (C) and (D) and prior to said step (E).
 23. Amethod according to claim 21 wherein: said cellulose ether is acarboxymethyl cellulose ether; said chromium compound is selected fromthe group consisting of ammonium chromate, ammonium dichromate, thealkali metal chromates and dichromates, chromium trioxide, and mixturesthereof; said reducing agent is selected from the group consisting ofhydroquinone, sodium sulfide, sodium hydrosulfite, sodium metabisulfite,potassium sulfite, sodium bisulfite, potassium metabisulfite, sodiumsulfite, sodium thiosulfate, ferrous sulfate, ferrous chloride,p-hydrazinobenzoic acid, hydrazine phosphite, hydrazine dihydrochloride,and mixtures thereof; and said acid is selected from the groupconsisting of hydrochloric acid, formic acid, acetic acid, and mixturesthereof.
 24. A method according to claim 23 wherein: said celluloseether is sodium carboxymethyl cellulose; said chromium compound issodium dichromate; and said reducing agent is sodium thiosulfate orsodium hydrosulfite.
 25. A method according to claim 24 wherein: saidcellulose ether is sodium carboxymethyl cellulose; said chromiumcompound is sodium dichromate; said reducing agent is sodium thiosulfateor sodium hydrosulfite; and said acid comprises a solution ofhydrochloric acid.